In general liquid crystal display devices, polarity inversion drive is performed in order to suppress liquid crystal deterioration and maintain display quality. However, inactive liquid crystal display devices, switching elements, such as TFTs (Thin-Film Transistors), which are provided for respective pixels, are characteristically insufficient to make the transmittance of the liquid crystal layer completely symmetrical with respect to positive and negative data voltages even if the polarities of video signals outputted by a video signal line driver circuit (also referred to as a “column electrode driver circuit” or a “data driver circuit”), which applies voltages to video signal lines (column electrodes) on the liquid crystal panel, are symmetrical, i.e., even if the polarities of applied voltages relative to the potential of a common electrode are symmetrical. Accordingly, in a polarity inversion drive scheme in which the polarity of a voltage applied to the liquid crystal is inverted (with respective to the potential of the common electrode) every frame (frame inversion drive scheme), flicker occurs in a displayed image on the liquid crystal panel (such flicker will be also referred to below as “flicker due to peak-to-peak asymmetry”). Recently, in particular, mobile information devices, such as cell phones, are required to have a high-quality display capability because of improvements to their processing performance and sophistication of their use, and therefore, such flicker due to peak-to-peak asymmetry becomes a problem. Accordingly, in a polarity inversion drive scheme employed for a liquid crystal module used in such a mobile information device, the polarity of an applied voltage is inverted every horizontal scanning signal line and also every frame (such a scheme is called a “line inversion drive scheme”). Moreover, in another polarity inversion drive scheme to be employed similarly, the polarity of an applied voltage is inverted every two vertically/horizontally adjacent pixels and also every frame (such a scheme is called a “dot inversion drive scheme”).
However, in the case where the line inversion drive scheme is employed, while high-quality display can be achieved, the frequency of polarity inversion of a video signal to be applied to the liquid crystal panel increases (the inversion frequency becomes higher), and the frequency at which to change the potential of the common electrode also becomes higher in order to reduce the voltage a driver IC (Integrated Circuit) is required to withstand. This results in increased power consumption. In addition, in the case where the dot inversion drive scheme is employed, inversion drive of the common electrode is not possible, so that the driver IC is required to withstand a higher voltage. This leads to increased device production cost and increased power consumption.
Therefore, in some drive schemes employed in recent years, the overall inversion frequency is reduced by providing scan stop periods in order not to change applied voltages for predetermined periods (see, for example, Japanese Laid-Open Patent Publication No. 2006-178435). By inserting such scan stop periods (hold off periods), it is rendered possible to meet the requirements for low power consumption in cell phones and suchlike.
The longer the scan stop period is set, the more power consumption is reduced, but during the scan stop period, current leakage occurs in capacitive elements, which are provided in pixel forming portions on the liquid crystal panel in order to hold applied voltages, so that the voltages to be held are reduced. As a result, the luminance of pixels to be displayed in accordance with the next voltages to be applied changes conspicuously (although the luminance should remain the same). Consequently, such luminance changes are visually recognized as flicker (such flicker will also be called “flicker due to current leakage” below).
Furthermore, during a scanning period, the pixel forming portions on the liquid crystal panel are sequentially selected row-by-row, and pixel voltages are applied thereto. At this time, a predetermined period of time (within a selected period) is taken until the pixel voltage of each pixel forming portion reaches the level of the voltage applied thereto, i.e., until data writing is completed, and if the voltage changes during that period by virtue of parasitic capacitance, the luminance of the displayed pixel changes as well. If such amounts of luminance change vary, for example, between frames, flicker might be visually recognized (such flicker will also be called “flicker due to data writing” below).
Furthermore, during the scanning period, data is written to selected pixel forming portions, and thereafter, scanning signal lines and video signal lines coupled to adjacent or neighboring pixel forming portions might change in potential, so that the applied and held voltage might change by virtue of parasitic capacitance created between (pixel electrodes at one end of) capacitive elements in the pixel forming portions and the signal lines (this phenomenon will also be called “drawing due to parasitic capacitance”). As a result, the luminance of pixels to be displayed in accordance with the next voltages to be applied changes conspicuously. Consequently, such luminance changes are visually recognized as flicker (such flicker will also be called “flicker due to drawing” below).
Note that the flicker due to data writing and the flicker due to drawing occur conspicuously in actuality because of differences in pixel voltages between two adjacent frames, which are caused when polarity inversion drive is performed, but they are described herein as being different from the aforementioned flicker due to peak-to-peak asymmetry.
In this regard, Japanese Laid-Open Patent Publication No. 2006-178435 discloses the configuration of a liquid crystal display device in which a backlight device is caused to blink on and off so fast as not to be visually detectable during both scanning periods and scan stop periods, and the duration for which the backlight device is kept off during the scanning period is set longer than the duration for which the device is kept on, thereby reducing flicker.